Pour your ice cream mix into the quart-sized plastic bag and shake. Then place the ice along with the rock salt in the gallon-sized bag and shake. Place the quart bag into the gallon bag. Wrap the gallon bag in a large towel and massage the bags, roll back and fourth across the top of your counter or a somewhat flat surface, or have the kids shake them up with a friend, until the ice cream thickens — a good 20 minutes. Don’t go wild on the massaging, rolling or shaking, because you don’t want either of the plastic bags to break.